The present invention pertains generally to electrostatic sound reproducers, and more particularly to an improved structure for a wire grid electrode electrostatic loudspeaker element, which results in improved performance and lower production costs.
In the art of high fidelity sound reproduction, the electrostatic loudspeaker has received wide recognition because of its excellent sound quality and smooth and faithful response over wide frequency ranges. In such speakers, a flexible sound producing diaphragm is positioned near an electrode, or in the case of a push-pull arrangement, a pair of electrodes, one on either side of the diaphragm. A DC polarization potential is applied between the diaphragm and the electrodes, and the audio signal is superimposed thereupon, causing the diaphragm to move in response thereto. Of course, in a push-pull arrangement, it is necessary that one or both of the electrodes be acoustically transparent so that the sound produced by the diaphragm can radiate outwardly through the electrode to the listening area. Usually some type of conductive screen or grid is used for the electrode. The individual conductive elements are close enough together to collectively define an electrostatic plane, and the spaces between the elements of the screen or grid provide apertures for the passage of sound produced by the diaphragm.
One important type of electrode construction is the wire grid electrode, wherein a plurality of spaced parallel wire segments supported by a frame form the electrode. Although the wire grid electrode has many advantages from a performance and manufacturing point of view, certain problems must be overcome in design and manufacture if the full performance of the electrostatic speaker is to be realized. One of these problems involves controlling the critical distance between the plane of the wire grid and the diaphragm, as this distance affects the relative sound output of the finished unit. Since the wires are inherently somewhat flexible, any slack or buckling of the wires could cause them to touch the diaphragm, which would result in severe distortion, and possibly dielectric breakdown of the wire insulation. Another problem involves allowing for thermal expansion of the various components so that the wires are not allowed to become too tight or too slack when temperature changes occur.
One proposed solution to these problems is set forth in U.S. Pat. No. 2,896,025, issued July 21, 1959. In that patent, a wire grid electrode is made up of a plurality of insulated wires lying in slots cut in a generally rectangular plane which has a plurality of web members defining sound passing apertures. One end of each of the wires is electrically connected to a common bus bar, while the other end of each wire is free. The insulation on each length of wire is glued to the wire guide slots, but the wire is free to move axially of the insulation due to temperature effects.
Although the structure described in the aforementioned patent does provide a high quality electrostatic loudspeaker element, it is subject to a number of problem areas which are overcome by the present invention. One problem is the difficulty of accurate control of the critical distance between the electrostatic plane defined by the wires, and the diaphragm. In the prior art apparatus represented by U.S. Pat. No. 2,896,025, it has been found that significant variations in this critical distance occur in manufacturing, causing variations in the sound output level from one unit to another, thus necessitating individual testing and matching. Another problem is that if any of the glue used to attach the wire insulation sheaths to the frame should happen to touch the free ends of the wires, the wire will be unable to slide back and forth during temperature changes, and this could cause the wire to buckle and touch the diaphragm. Another problem is that a number of the manufacturing steps, such as individually soldering the wire elements to the bus bar, and applying glue to the wires in the slots take an undue amount of time and result in a high production cost.
The present invention provides an improved wire grid electrostatic speaker element which overcomes these and other problems existing in the prior art. In the present invention, the frame is molded from a carefully chosen material having thermal expansion which equals that of the wire. The wire is thus wound continuously around the frame without any need for cutting or soldering to a bus bar. Accurate temperature compensation and reduced production costs are thereby achieved. Further, a dielectric spacer element is positioned between the wire grid and the diaphragm to accurately control the critical distance therebetween. In the preferred embodiment, the spacer element is double coated with a pressure sensitive adhesive for ease in assembly. In addition to lower production costs, the present invention provides an electrostatic dust shielding effect which keeps dust out of the diaphragm area.